Electronic color imaging systems generally capture three distinct spectral components of an image, for example, red, green and blue. Each of these components is represented by an electrical signal. In many applications, the separate electrical signals are sampled and digitized. If the image is to be stored, the digital samples may be written into a digital memory. The image may be reproduced by applying the three signals to a device which combines the colors represented by the three signals.
Sequential color imaging systems are well known. In these systems, a single image sensing device sequentially receives, for example, red, blue and green color information at a high rate relative to the rate at which the image is changing. If a document is moving during the imaging operation, The system must operate at relatively high frequencies since the document must be resolved into three distinct signals while it is held momentarily still.
Other existing systems use a single linear or time delay and integration (TDI) imaging device to capture three separate scans of a document, each taken with a separate filter in place near the lens. The imaging device generally resolves lines of picture elements (pels) and scans the image incrementally, line by line. Either the document, the entire system except the document, the imaging device or a system of mirrors may be moved to provide the scan.
The primary disadvantage of a system of this type is the time required for three scans at a given maximum output data rate. If the document must move while it is being scanned, it may be necessary to make three passes through the system to obtain all three images. These passes add to the time required to process the document and may present undesirable alignment problems.
One method of avoiding multiple passes is to use three separately packaged imaging devices together with a system of spectrally selective beam splitters and filters. In this system, a different spectral band is applied to each device. These systems are disadvantageous because they require expensive optical components and need precise alignment.
Multiple passes over the image may also be avoided by placing three imaging devices close together on a single chip or substrate arranged such that each device is exposed to light in a different spectral band. This may be done in several ways. According to a first method, the light from the illuminated image may be dispersed so that the different devices are simultaneously exposed to different spectral bands. This method requires a linear filament illuminator and expensive optical components. Moreover, this method makes inefficient use of its illuminator and is not suitable for scanners which move only the imaging device.
A second method employs a uniform illuminator but places different filters over the different imaging devices. This method is disadvantageous since it requires special technology to apply and align the filters to the devices.
A paper by Yao et al. entitled "A Spatial Image Separator for Color Scanning" SPIE Vol 809--Scanning Imaging Technology pp 52-54, March 1987, describes a single-pass TDI color imager which uses a single TDI array. This system requires relatively complex and, thus, expensive optical components.
U.S. Pat. No. 4,500,914 to Watanabe et al. relates to a color imaging array in which red, green and blue sensor elements are defined by a single charge coupled device (CCD) imaging array that is tessellated with respective red, green and blue filter elements.
U.S. Pat. No. 4,264,921 to Pennington et al. relates to a single-pass color imager having three TDI arrays which each receive different spectral illumination.
U.S. Pat. No. 4,628,350 to Aughton et al. concerns an imaging system in which a light beam is passed through a moving transparency, through a rotating filter element and onto a single linear imaging device. The rotating filter element sequentially passes light in three distinct spectral components. The light is converted into electrical signals by the single imaging device.
Aughton teaches the use of a single linear imaging device with rapidly changing spectral image components. The advantages of TDI imaging arrays over linear imagers are well known: higher effective sensitivity to light and greater spatial uniformity and fidelity in the captured image. In a single-pass imager, however, TDI arrays cannot be substituted for linear arrays in a straightforward manner. This is because TDI imaging arrays operate in a pipelined mode, containing an electronic representation of several image pel lines at all times. Thus, if known TDI arrays were substituted for the linear arrays, the three color images would be mixed together, preventing color image reproduction from the electronic output.
It is therefore an object of the present invention to combine the advantages of a single-pass color imager with those of TDI imagers.